Solution by Cameron Powell1 is the best answer:
- works the same in classic and new outlook, and it simplifies a lot the process of using full templates with prefilled Subject and Recipients
- just double click on the .oft file.
The concept is so simple it is surprising it is not available in Outlook online, nor explained in standard documentation instead of the insane digging through several menus and dialogs (New Items/More items/Choose Form/User Tenplates in File System...)
https://support.microsoft.com/en-us/office/send-an-email-message-based-on-a-template-56c645fc-1b25-4059-808b-55ee72b6bc2d.
Simple trick was there all this time, I never figured it out, thanks Cameron.
Note that by dragging the .oft file to outlook icon in the taskbar you can pin it for easy access with right click if you only have a few templates.